For example. In KCNO, the mag variation on sectional chart is 11.5E. But on some websites and in Garmin pilot, it says it's 14E. Why would this happen?
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2 Answers
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Relating to the U.S.:
The short answer:
The magnetic variation published in the FAA database and shown on the Garmin page for Chino Airport (KCNO) is based on the magnetic variation as it existed in 1980. The actual magnetic variation shown on the sectional chart as a magenta isogonic line varies/changes over time and is revised, as necessary, every 5 years.
The magnetic variation for some airports or navaids in the official database have never been revised since their original variation was determined (depending on how old the airport or navaid is). In some cases the last revision or original determination/publication of the magnetic variation could be more than 50 years old.
The longer answer:
Your Garmin pilot (image from your question) shows the same information that is in the official database. The magnetic variation of Chino (14E) was last determined in 1980 (see the image below):
This is the same as shown on your Garmin equipment (14E). The magnetic variation shown on the sectional chart for Chino is about 11.5E (shown by the magenta isogonic line closest to the airport).
Note the Paradise VORTAC (see the second image below), which is about 6 nm SE from Chino, shows a magnetic variation of (15E) and was measured/determined in 1965. According to this FAA information with respect to navigational aids (under the first section of "Visual Charts"):
"...readjustments to match the isogonic value require a total shut down of the equipment, plus recertification and flight check verification. This process begins when a navigational aid is out of tolerance by at least +/-6 degrees."
The magnetic variation of the earth changes all of the time. The isogonic lines published on a Sectional Chart are updated every 5 years. See the third image below from the Aeronautical Chart Users Guide, page 32.
There are likely thousands of airports and navigational aids that will show in the official database a different magnetic variation (which changes all of the time and to varying degrees based on location) than actually exists for that location and shown on the sectional chart as a magenta isogonic line. The magnetic variation for some airports/navaids may have never been updated since originally determined/published (depending on how old the airport or navaid is). In some cases the last revision or original determination/publication of the magnetic variation could be more than 50 years old.
Lastly, here is a good explanation from "IFR Magazine" entitled "Mag Course mismatches" that may help you understand why the magnetic variation (isogonic lines) shown on the chart might be different from what is currently in the official database.
(all emphasis above is mine)
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Typically, the data is taken from the respective national aviation authority. The FAA has incorrect information for KCNO in its database.
https://www.faa.gov/air_traffic/flight_info/aeronav/aero_data/NASR_Subscription/
Click on the current 28 Day NASR subscription.
Download the Airports and other Landing Facities zp file
Open the APT_BASE.csv file
CNO shows a magnetic variation of 14E reported in 1980.
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$\begingroup$ How can this be reported to the FAA? $\endgroup$– SomeoneCommented Aug 19, 2023 at 15:43
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1$\begingroup$ faa.gov/air_traffic/flight_info/aeronav/aero_data $\endgroup$– wbeard52Commented Aug 19, 2023 at 16:35